The annual cost of corrosion for the U.S. was estimated to be $276 billion or approximately 3.1% of the U.S. Gross Domestic Product. Besides these costs, corrosion also adversely impacts safety and efficiency in a number of economic sectors such as transportation and infrastructure. Aging infrastructure has been deemed “one of the most serious problems faced by society today,” in a study performed by the Federal Highway Administration (FHWA). As aging infrastructure and transportation vehicles approach the end of their design lifetime, proper asset management is increasingly important in an effort to safely extend service lifetimes. However, due to the complex nature of these structures, corrosion often occurs in areas that are difficult to access, making regular maintenance checks impossible. Therefore, corrosion maintenance practices are schedule-based, where the extent of material and corrosion damage is examined after a certain number of hours or set calendar service periods. The removal of material to determine the degree of material degradation and corrosion damage is costly and destructive, and although some inspections reveal significant damage, other inspections uncover no damage, which indicates the structure or component had significant remaining service life.
Increasingly, there is a need to transition from these current schedule-based maintenance practices to condition-based practices, where damage is addressed once it reaches a critical level and before a potentially disabling event occurs. Monitoring systems and models predicting future damage have been developed and installed across platforms for corrosion control. In applications where the structural alloys or substrates are difficult to access (structures with multilayer coating and coverings, insulation, or embedded in concrete), however, online monitoring systems are still challenging to implement. For many known monitoring systems, a direct electrical connection to the structure is necessary for performing electrochemical measurements, such as linear polarization or electrochemical impedance spectroscopy, to determine the progression of material degradation and corrosion. This direct electrical connection must be attached to the structure as it is built or it can be added in later. Direct electrical connection between the sensing elements and structure may require penetrations through protective coatings that can then become failure points or otherwise impair the functionality of the protection system. In many applications where the structure is difficult to access, electrically connecting directly to the substrate is impossible due to the possibility of electrical interference or wires through surrounding material becoming failure points.
The use of wireless power and signal transfer to excite the corrosion and coating condition sensors eliminates the need for conductors or physical connectors that penetrate through the coatings, covers, or insulation layers or the structure. Elimination of wired power and communications interfaces also eliminates discontinuities that may become failure points or otherwise impair the functionality of the protective coverings. Finally, wireless power and communications are useful for monitoring moving parts and components such as a propulsion shaft for a ship.
Electrochemical techniques including linear polarization or electrochemical impedance spectroscopy may be used in some monitoring systems for detecting corrosion and coating condition. These techniques can be used to track with time the evolution of barrier properties of coatings and polarization resistance of the metallic structure, which is then converted to a corrosion rate. When these techniques are performed with a two-electrode or three-electrode setup (working, counter, and reference electrode), a direct electrical connection to the structure is required. For a structure that is difficult to access, such as buried in concrete or soil, if the connection was not placed when it was first constructed, it can be extremely costly and difficult to later retrofit an electrical connection to the buried structure. In many cases, it can be impossible to electrically connect to the structure because of the possibility of interference with normal operations of the structure and damage to the coatings, covers, or insulation surrounding it.
Electrochemical measurement techniques also require physical electrical connections between the sensing elements and the power and communications interface. The requirement for physical electrical connections means that batteries or line-power must be available or incorporated in the design. The size of the electronics, batteries, or wiring required for these connections creates difficulties for installing and maintaining embedded systems. In the case of battery powered monitoring systems, battery replacement becomes a routine maintenance burden. For line-powered systems the electrical connection needs to be routed to a power source, and the connections to the embedded sensors need to penetrate through the structure, coatings, covering, or insulation. For moving components these techniques may be impossible or require complex connections, e.g., a slip ring connection, that also introduce complexity, reduce reliability, and increase maintenance activities.
Other nondestructive techniques to monitor a buried structure or a free-standing coated structure and detect both material degradation surrounding the structure and corrosion of the structure itself include acoustic emission, ultrasonic, electromagnetic, thermographic, optical fiber, or radiographic methods. Although these techniques might be effective in detecting corrosion damage over a wide area, they can only detect damage that is on the size order of large pits or cracks and that often has already reached a critical level. The early stages of corrosion that include the breakdown of the passive oxide layer and initiation of corrosion are not detectable using these techniques. They can also be useful for detecting coating degradation but are unable to distinguish degradation between layers in a multilayer coating stack-up.
“Coupons” that are made from the same material as the structure may be buried or placed near the structure of interest in combination with a monitoring method. These coupons may then be retrieved at various time intervals and examined for corrosion damage. Although coupons can provide a qualitative indication of the condition of the structure, they do not provide direct quantitative data on the health of the structure and are costly to place and remove.
Surrogate sensing methods could measure coating barrier properties, free corrosion, and environmental properties by placing electrodes fabricated from the same metallic material as the structure near the structure of interest and collecting electrochemical and environmental measurements. Although such methods could provide an indicator of potential corrosion damage on the structure, they cannot be embedded, are only indirect measures of conditions, and are impossible to implement in applications where the structure is inaccessible.
There is a need for an in situ (e.g., on site, in place, local, etc.) apparatus, system, and method of detecting electrochemical corrosion and material or coating degradation that does not require electrical connection to the structure and can directly detect the earliest stages of coating breakdown and corrosion before damage progresses to more serious cases, e.g., pitting and cracking. It would be desirable to also distinguish between coating degradation of layers in a multilayer coating stack-up. Furthermore, it would be desirable for such a corrosion and coating condition measurement system to use wireless power and data transfer. Such technology would enable condition-based maintenance, reduce the number of costly destructive inspections, decrease the risk of a failure between inspection cycles, and support service life extension based on condition.
Example embodiments include a sensing apparatus for sensing corrosion condition information. A pair of contactless electrodes are placed on a coating on a surface of a structure or within or between one or more coatings on or over the surface of the structure. The term “embedded” as used herein includes being placed on a coating on a surface of a structure or within or between one or more coatings on or over the surface of the structure. The embedded electrodes are configured to function as sensors when activated to generate a current flow through the electrodes at multiple different frequencies.
Contactless control circuitry for co-location with the embedded electrodes is configured, when the electrodes and electronic circuitry are wirelessly activated, to transmit impedance data generated from the activated electrodes for detection by a receiver external to the structure. The impedance data is associated with corrosion and/or coating condition information of the structure and/or the one or more coatings.
In example applications, the sensing apparatus includes excitation circuitry for co-location with the embedded electrodes and configured to activate the pair of electrodes as sensors by applying a voltage potential at one or multiple frequencies between the pair of electrodes so as to excite and measure current flow between the electrodes and convert the current flow between the electrodes to an impedance measurement. The contactless control circuitry is configured to control excitation circuitry and to transmit digital impedance data, based on the impedance measurement, to an external, preferably wireless receiver.
An array of multiple pairs of contactless embedded electrodes may be placed on a coating on a surface of a structure or within or between one or more coatings on or over the surface of the structure. For such an array, the control circuitry may include a multiplexer configured to obtain measurements across any combination of pairs of electrodes.
In example applications, the control circuitry of the sensing apparatus includes a power source, e.g., an ultracapacitor, and radio frequency (RF) interface and communications circuitry configured to receive RF power and to charge the power source, e.g., ultracapacitor. The excitation circuitry and contactless control circuitry are configured to be powered by the charged power source. The contactless control circuitry is configured to provide digital data representing the impedance data to the RF interface and communications circuitry, and the RF interface and communications circuitry is configured to transmit the digital data to a data collection system over a radio interface.
The corrosion condition information may include, for example, a damage state parameter determined from a dry state impedance and a current impedance detected using the electrodes. The corrosion condition information may include an absolute or relative change in corrosion of the structure and/or the one or more coatings based on the impedance data associated with the pair of electrodes at multiple different frequencies.
The pair of contactless embedded electrodes is not connected to the structure through a direct electrical, optical, or other type of signal connection.
In example applications, the embedded electrodes in the pair are placed at a predetermined distance from each other so that current flow generated with the pair of electrodes is activated primarily through the structure and/or the one or more coatings.
In example embodiments, the sensing apparatus further includes one or more addition of nondestructive monitoring devices.
Other example embodiments include a sensing system for sensing corrosion condition information having a sensing device that includes a pair of contactless embedded electrodes for placement on a coating on a surface of a structure or embeddable within or between one or more coatings on or over the surface of the structure. Contactless circuitry is provided for co-location with the embedded electrodes and has communications circuitry for wireless communication. The sensing system further includes a data collection system, external and separated from the structure and any coatings on or over the structure, configured to wirelessly communicate with the sensing device and including a power source, control circuitry, and interface and communication circuitry. The pair of contactless electrodes is configured to function as sensors when activated to excite current flow between the electrodes at multiple different frequencies. The co-located contactless circuitry is configured to detect and convert the current flow to impedance data and wirelessly transmit the impedance data to the interface and communication circuitry of the data collection system. The impedance data is associated with a corrosion condition of the structure and/or the one or more coatings.
In example embodiments, the data collection system may be configured to transmit data and/or power via the interface and communication circuitry over a radio frequency (RF) interface to the sensing device.
Example applications include a data display system coupled to the data collection system that includes a graphical user interface configured to communicate from or to a user information regarding the coating and/or corrosion condition of the structure.
Other example embodiments include a method of sensing corrosion condition information. The method includes: locating or embedding a sensing device on a coating on a surface of a structure or within or between one or more coatings on or over the surface of the structure. The sensing device includes a pair of contactless electrodes. In some example applications, the sensing device also has co-located wireless communications and control circuitry. The pair of contactless electrodes is activated to excite current flow between the electrodes at one or more different frequencies. The control circuitry co-located or otherwise detects and converts the current flow to impedance data. The impedance data is associated with a corrosion condition of the structure and/or the one or more coatings.
In example embodiments, the sensing device transmits the impedance data to a data collection system, e.g., over a radio frequency (RF) interface. The data collection system is also external and separated from the structure and any coatings on or over the structure.
Other example embodiments include apparatus and methods of sensing corrosion condition information using a “walk-up” sensor device that is placed on a coating on a surface of a structure. The sensing device includes a pair of contactless electrodes and control circuitry that excite current flow between the electrodes at one or multiple frequencies. The control circuitry detects and converts the current flow to impedance data and provides the impedance data to a data collection system. The impedance data is associated with a corrosion condition of the structure and/or the one or more coatings.